Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.80
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
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20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
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26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.80 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.79 2008/01/01 09:27:33 dtucker Exp $
! 38: .Dd $Mdocdate: January 1 2008 $
1.1 stevesk 39: .Dt SSHD_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm sshd_config
43: .Nd OpenSSH SSH daemon configuration file
44: .Sh SYNOPSIS
1.71 jmc 45: .Nm /etc/ssh/sshd_config
1.1 stevesk 46: .Sh DESCRIPTION
1.53 jmc 47: .Xr sshd 8
1.1 stevesk 48: reads configuration data from
49: .Pa /etc/ssh/sshd_config
50: (or the file specified with
51: .Fl f
52: on the command line).
53: The file contains keyword-argument pairs, one per line.
54: Lines starting with
55: .Ql #
56: and empty lines are interpreted as comments.
1.56 dtucker 57: Arguments may optionally be enclosed in double quotes
58: .Pq \&"
59: in order to represent arguments containing spaces.
1.1 stevesk 60: .Pp
61: The possible
62: keywords and their meanings are as follows (note that
63: keywords are case-insensitive and arguments are case-sensitive):
64: .Bl -tag -width Ds
1.30 djm 65: .It Cm AcceptEnv
66: Specifies what environment variables sent by the client will be copied into
67: the session's
68: .Xr environ 7 .
69: See
70: .Cm SendEnv
71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.31 djm 74: Note that environment passing is only supported for protocol 2.
1.30 djm 75: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 76: .Ql *
1.30 djm 77: and
78: .Ql \&? .
1.31 djm 79: Multiple environment variables may be separated by whitespace or spread
1.30 djm 80: across multiple
81: .Cm AcceptEnv
82: directives.
1.31 djm 83: Be warned that some environment variables could be used to bypass restricted
1.30 djm 84: user environments.
85: For this reason, care should be taken in the use of this directive.
86: The default is not to accept any environment variables.
1.37 djm 87: .It Cm AddressFamily
88: Specifies which address family should be used by
1.53 jmc 89: .Xr sshd 8 .
1.37 djm 90: Valid arguments are
91: .Dq any ,
92: .Dq inet
1.52 jmc 93: (use IPv4 only), or
1.37 djm 94: .Dq inet6
95: (use IPv6 only).
96: The default is
97: .Dq any .
1.1 stevesk 98: .It Cm AllowGroups
99: This keyword can be followed by a list of group name patterns, separated
100: by spaces.
101: If specified, login is allowed only for users whose primary
102: group or supplementary group list matches one of the patterns.
103: Only group names are valid; a numerical group ID is not recognized.
104: By default, login is allowed for all groups.
1.54 jmc 105: The allow/deny directives are processed in the following order:
106: .Cm DenyUsers ,
107: .Cm AllowUsers ,
108: .Cm DenyGroups ,
109: and finally
110: .Cm AllowGroups .
1.49 jmc 111: .Pp
112: See
113: .Sx PATTERNS
114: in
115: .Xr ssh_config 5
116: for more information on patterns.
1.1 stevesk 117: .It Cm AllowTcpForwarding
118: Specifies whether TCP forwarding is permitted.
119: The default is
120: .Dq yes .
121: Note that disabling TCP forwarding does not improve security unless
122: users are also denied shell access, as they can always install their
123: own forwarders.
124: .It Cm AllowUsers
125: This keyword can be followed by a list of user name patterns, separated
126: by spaces.
1.14 jmc 127: If specified, login is allowed only for user names that
1.1 stevesk 128: match one of the patterns.
129: Only user names are valid; a numerical user ID is not recognized.
130: By default, login is allowed for all users.
131: If the pattern takes the form USER@HOST then USER and HOST
132: are separately checked, restricting logins to particular
133: users from particular hosts.
1.54 jmc 134: The allow/deny directives are processed in the following order:
135: .Cm DenyUsers ,
136: .Cm AllowUsers ,
137: .Cm DenyGroups ,
138: and finally
139: .Cm AllowGroups .
1.49 jmc 140: .Pp
141: See
142: .Sx PATTERNS
143: in
144: .Xr ssh_config 5
145: for more information on patterns.
1.1 stevesk 146: .It Cm AuthorizedKeysFile
147: Specifies the file that contains the public keys that can be used
148: for user authentication.
149: .Cm AuthorizedKeysFile
150: may contain tokens of the form %T which are substituted during connection
1.52 jmc 151: setup.
1.17 jmc 152: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 153: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 154: %u is replaced by the username of that user.
155: After expansion,
156: .Cm AuthorizedKeysFile
157: is taken to be an absolute path or one relative to the user's home
158: directory.
159: The default is
160: .Dq .ssh/authorized_keys .
161: .It Cm Banner
162: The contents of the specified file are sent to the remote user before
163: authentication is allowed.
1.78 djm 164: If the argument is
165: .Dq none
166: then no banner is displayed.
1.1 stevesk 167: This option is only available for protocol version 2.
168: By default, no banner is displayed.
169: .It Cm ChallengeResponseAuthentication
1.50 jmc 170: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 171: All authentication styles from
172: .Xr login.conf 5
173: are supported.
174: The default is
175: .Dq yes .
1.80 ! djm 176: .It Cm ChrootDirectory
! 177: Specifies a path to
! 178: .Xr chroot 2
! 179: to after authentication.
! 180: This path, and all its components, must be root-owned directories that are
! 181: not writable by any other user or group.
! 182: .Pp
! 183: The path may contain the following tokens that are expanded at runtime once
! 184: the connecting user has been authenticated: %% is replaced by a literal '%',
! 185: %h is replaced by the home directory of the user being authenticated, and
! 186: %u is replaced by the username of that user.
! 187: .Pp
! 188: The
! 189: .Cm ChrootDirectory
! 190: must contain the necessary files and directories to support the
! 191: users' session.
! 192: For an interactive session this requires at least a shell, typically
! 193: .Xr sh 1 ,
! 194: and basic
! 195: .Pa /dev
! 196: nodes such as
! 197: .Xr null 4 ,
! 198: .Xr zero 4 ,
! 199: .Xr stdin 4 ,
! 200: .Xr stdout 4 ,
! 201: .Xr stderr 4 ,
! 202: .Xr arandom 4
! 203: and
! 204: .Xr tty 4
! 205: devices.
! 206: For file transfer sessions using
! 207: .Dq sftp ,
! 208: no additional configuration of the environment is necessary if the
! 209: in-process sftp server is used (see
! 210: .Cm Subsystem
! 211: for details.
! 212: .Pp
! 213: The default is not to
! 214: .Xr chroot 2 .
1.1 stevesk 215: .It Cm Ciphers
216: Specifies the ciphers allowed for protocol version 2.
217: Multiple ciphers must be comma-separated.
1.34 dtucker 218: The supported ciphers are
219: .Dq 3des-cbc ,
220: .Dq aes128-cbc ,
221: .Dq aes192-cbc ,
222: .Dq aes256-cbc ,
223: .Dq aes128-ctr ,
224: .Dq aes192-ctr ,
225: .Dq aes256-ctr ,
1.43 djm 226: .Dq arcfour128 ,
227: .Dq arcfour256 ,
1.34 dtucker 228: .Dq arcfour ,
229: .Dq blowfish-cbc ,
230: and
231: .Dq cast128-cbc .
1.52 jmc 232: The default is:
233: .Bd -literal -offset 3n
234: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
235: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
236: aes192-ctr,aes256-ctr
1.1 stevesk 237: .Ed
238: .It Cm ClientAliveCountMax
1.48 jmc 239: Sets the number of client alive messages (see below) which may be
1.1 stevesk 240: sent without
1.52 jmc 241: .Xr sshd 8
1.17 jmc 242: receiving any messages back from the client.
243: If this threshold is reached while client alive messages are being sent,
1.52 jmc 244: sshd will disconnect the client, terminating the session.
1.17 jmc 245: It is important to note that the use of client alive messages is very
246: different from
1.27 markus 247: .Cm TCPKeepAlive
1.17 jmc 248: (below).
249: The client alive messages are sent through the encrypted channel
250: and therefore will not be spoofable.
251: The TCP keepalive option enabled by
1.27 markus 252: .Cm TCPKeepAlive
1.17 jmc 253: is spoofable.
254: The client alive mechanism is valuable when the client or
1.1 stevesk 255: server depend on knowing when a connection has become inactive.
256: .Pp
1.17 jmc 257: The default value is 3.
258: If
1.1 stevesk 259: .Cm ClientAliveInterval
1.48 jmc 260: (see below) is set to 15, and
1.1 stevesk 261: .Cm ClientAliveCountMax
1.52 jmc 262: is left at the default, unresponsive SSH clients
1.1 stevesk 263: will be disconnected after approximately 45 seconds.
1.57 markus 264: This option applies to protocol version 2 only.
1.42 djm 265: .It Cm ClientAliveInterval
266: Sets a timeout interval in seconds after which if no data has been received
267: from the client,
1.52 jmc 268: .Xr sshd 8
1.42 djm 269: will send a message through the encrypted
270: channel to request a response from the client.
271: The default
272: is 0, indicating that these messages will not be sent to the client.
273: This option applies to protocol version 2 only.
1.3 markus 274: .It Cm Compression
1.44 markus 275: Specifies whether compression is allowed, or delayed until
276: the user has authenticated successfully.
1.3 markus 277: The argument must be
1.44 markus 278: .Dq yes ,
279: .Dq delayed ,
1.3 markus 280: or
281: .Dq no .
282: The default is
1.44 markus 283: .Dq delayed .
1.1 stevesk 284: .It Cm DenyGroups
285: This keyword can be followed by a list of group name patterns, separated
286: by spaces.
287: Login is disallowed for users whose primary group or supplementary
288: group list matches one of the patterns.
289: Only group names are valid; a numerical group ID is not recognized.
290: By default, login is allowed for all groups.
1.54 jmc 291: The allow/deny directives are processed in the following order:
292: .Cm DenyUsers ,
293: .Cm AllowUsers ,
294: .Cm DenyGroups ,
295: and finally
296: .Cm AllowGroups .
1.49 jmc 297: .Pp
298: See
299: .Sx PATTERNS
300: in
301: .Xr ssh_config 5
302: for more information on patterns.
1.1 stevesk 303: .It Cm DenyUsers
304: This keyword can be followed by a list of user name patterns, separated
305: by spaces.
306: Login is disallowed for user names that match one of the patterns.
307: Only user names are valid; a numerical user ID is not recognized.
308: By default, login is allowed for all users.
309: If the pattern takes the form USER@HOST then USER and HOST
310: are separately checked, restricting logins to particular
311: users from particular hosts.
1.54 jmc 312: The allow/deny directives are processed in the following order:
313: .Cm DenyUsers ,
314: .Cm AllowUsers ,
315: .Cm DenyGroups ,
316: and finally
317: .Cm AllowGroups .
1.49 jmc 318: .Pp
319: See
320: .Sx PATTERNS
321: in
322: .Xr ssh_config 5
323: for more information on patterns.
1.67 dtucker 324: .It Cm ForceCommand
325: Forces the execution of the command specified by
326: .Cm ForceCommand ,
327: ignoring any command supplied by the client.
328: The command is invoked by using the user's login shell with the -c option.
329: This applies to shell, command, or subsystem execution.
330: It is most useful inside a
331: .Cm Match
332: block.
333: The command originally supplied by the client is available in the
334: .Ev SSH_ORIGINAL_COMMAND
335: environment variable.
1.1 stevesk 336: .It Cm GatewayPorts
337: Specifies whether remote hosts are allowed to connect to ports
338: forwarded for the client.
339: By default,
1.52 jmc 340: .Xr sshd 8
1.15 jmc 341: binds remote port forwardings to the loopback address.
342: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 343: .Cm GatewayPorts
1.52 jmc 344: can be used to specify that sshd
1.39 djm 345: should allow remote port forwardings to bind to non-loopback addresses, thus
346: allowing other hosts to connect.
347: The argument may be
348: .Dq no
349: to force remote port forwardings to be available to the local host only,
1.1 stevesk 350: .Dq yes
1.39 djm 351: to force remote port forwardings to bind to the wildcard address, or
352: .Dq clientspecified
353: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 354: The default is
355: .Dq no .
1.23 markus 356: .It Cm GSSAPIAuthentication
1.25 markus 357: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 358: The default is
1.23 markus 359: .Dq no .
360: Note that this option applies to protocol version 2 only.
361: .It Cm GSSAPICleanupCredentials
362: Specifies whether to automatically destroy the user's credentials cache
363: on logout.
364: The default is
365: .Dq yes .
366: Note that this option applies to protocol version 2 only.
1.1 stevesk 367: .It Cm HostbasedAuthentication
368: Specifies whether rhosts or /etc/hosts.equiv authentication together
369: with successful public key client host authentication is allowed
1.50 jmc 370: (host-based authentication).
1.1 stevesk 371: This option is similar to
372: .Cm RhostsRSAAuthentication
373: and applies to protocol version 2 only.
1.70 dtucker 374: The default is
375: .Dq no .
376: .It Cm HostbasedUsesNameFromPacketOnly
377: Specifies whether or not the server will attempt to perform a reverse
378: name lookup when matching the name in the
379: .Pa ~/.shosts ,
380: .Pa ~/.rhosts ,
381: and
382: .Pa /etc/hosts.equiv
383: files during
384: .Cm HostbasedAuthentication .
385: A setting of
386: .Dq yes
387: means that
388: .Xr sshd 8
389: uses the name supplied by the client rather than
390: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 391: The default is
392: .Dq no .
393: .It Cm HostKey
394: Specifies a file containing a private host key
395: used by SSH.
396: The default is
397: .Pa /etc/ssh/ssh_host_key
398: for protocol version 1, and
399: .Pa /etc/ssh/ssh_host_rsa_key
400: and
401: .Pa /etc/ssh/ssh_host_dsa_key
402: for protocol version 2.
403: Note that
1.52 jmc 404: .Xr sshd 8
1.1 stevesk 405: will refuse to use a file if it is group/world-accessible.
406: It is possible to have multiple host key files.
407: .Dq rsa1
408: keys are used for version 1 and
409: .Dq dsa
410: or
411: .Dq rsa
412: are used for version 2 of the SSH protocol.
413: .It Cm IgnoreRhosts
414: Specifies that
415: .Pa .rhosts
416: and
417: .Pa .shosts
418: files will not be used in
419: .Cm RhostsRSAAuthentication
420: or
421: .Cm HostbasedAuthentication .
422: .Pp
423: .Pa /etc/hosts.equiv
424: and
425: .Pa /etc/shosts.equiv
426: are still used.
427: The default is
428: .Dq yes .
429: .It Cm IgnoreUserKnownHosts
430: Specifies whether
1.52 jmc 431: .Xr sshd 8
1.1 stevesk 432: should ignore the user's
1.41 djm 433: .Pa ~/.ssh/known_hosts
1.1 stevesk 434: during
435: .Cm RhostsRSAAuthentication
436: or
437: .Cm HostbasedAuthentication .
438: The default is
439: .Dq no .
440: .It Cm KerberosAuthentication
1.24 markus 441: Specifies whether the password provided by the user for
1.1 stevesk 442: .Cm PasswordAuthentication
1.24 markus 443: will be validated through the Kerberos KDC.
1.1 stevesk 444: To use this option, the server needs a
445: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 446: The default is
1.29 dtucker 447: .Dq no .
448: .It Cm KerberosGetAFSToken
1.45 djm 449: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 450: an AFS token before accessing the user's home directory.
1.52 jmc 451: The default is
1.1 stevesk 452: .Dq no .
453: .It Cm KerberosOrLocalPasswd
1.52 jmc 454: If password authentication through Kerberos fails then
1.1 stevesk 455: the password will be validated via any additional local mechanism
456: such as
457: .Pa /etc/passwd .
1.52 jmc 458: The default is
1.1 stevesk 459: .Dq yes .
460: .It Cm KerberosTicketCleanup
461: Specifies whether to automatically destroy the user's ticket cache
462: file on logout.
1.52 jmc 463: The default is
1.1 stevesk 464: .Dq yes .
465: .It Cm KeyRegenerationInterval
466: In protocol version 1, the ephemeral server key is automatically regenerated
467: after this many seconds (if it has been used).
468: The purpose of regeneration is to prevent
469: decrypting captured sessions by later breaking into the machine and
470: stealing the keys.
471: The key is never stored anywhere.
472: If the value is 0, the key is never regenerated.
473: The default is 3600 (seconds).
474: .It Cm ListenAddress
475: Specifies the local addresses
1.52 jmc 476: .Xr sshd 8
1.1 stevesk 477: should listen on.
478: The following forms may be used:
479: .Pp
480: .Bl -item -offset indent -compact
481: .It
482: .Cm ListenAddress
483: .Sm off
484: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
485: .Sm on
486: .It
487: .Cm ListenAddress
488: .Sm off
489: .Ar host No | Ar IPv4_addr No : Ar port
490: .Sm on
491: .It
492: .Cm ListenAddress
493: .Sm off
494: .Oo
495: .Ar host No | Ar IPv6_addr Oc : Ar port
496: .Sm on
497: .El
498: .Pp
499: If
500: .Ar port
501: is not specified,
1.52 jmc 502: sshd will listen on the address and all prior
1.1 stevesk 503: .Cm Port
1.17 jmc 504: options specified.
505: The default is to listen on all local addresses.
1.15 jmc 506: Multiple
1.1 stevesk 507: .Cm ListenAddress
1.17 jmc 508: options are permitted.
509: Additionally, any
1.1 stevesk 510: .Cm Port
1.52 jmc 511: options must precede this option for non-port qualified addresses.
1.1 stevesk 512: .It Cm LoginGraceTime
513: The server disconnects after this time if the user has not
514: successfully logged in.
515: If the value is 0, there is no time limit.
1.12 stevesk 516: The default is 120 seconds.
1.1 stevesk 517: .It Cm LogLevel
518: Gives the verbosity level that is used when logging messages from
1.53 jmc 519: .Xr sshd 8 .
1.1 stevesk 520: The possible values are:
1.52 jmc 521: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 522: The default is INFO.
523: DEBUG and DEBUG1 are equivalent.
524: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
525: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 526: .It Cm MACs
527: Specifies the available MAC (message authentication code) algorithms.
528: The MAC algorithm is used in protocol version 2
529: for data integrity protection.
530: Multiple algorithms must be comma-separated.
1.52 jmc 531: The default is:
1.77 jmc 532: .Bd -literal -offset indent
533: hmac-md5,hmac-sha1,umac-64@openssh.com,
534: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
535: .Ed
1.60 dtucker 536: .It Cm Match
1.61 jmc 537: Introduces a conditional block.
1.65 dtucker 538: If all of the criteria on the
1.60 dtucker 539: .Cm Match
1.65 dtucker 540: line are satisfied, the keywords on the following lines override those
541: set in the global section of the config file, until either another
1.60 dtucker 542: .Cm Match
1.65 dtucker 543: line or the end of the file.
1.61 jmc 544: The arguments to
1.60 dtucker 545: .Cm Match
1.65 dtucker 546: are one or more criteria-pattern pairs.
1.60 dtucker 547: The available criteria are
548: .Cm User ,
1.69 dtucker 549: .Cm Group ,
1.60 dtucker 550: .Cm Host ,
551: and
552: .Cm Address .
553: Only a subset of keywords may be used on the lines following a
554: .Cm Match
555: keyword.
556: Available keywords are
1.62 dtucker 557: .Cm AllowTcpForwarding ,
1.72 dtucker 558: .Cm Banner ,
1.67 dtucker 559: .Cm ForceCommand ,
1.62 dtucker 560: .Cm GatewayPorts ,
1.72 dtucker 561: .Cm GSSApiAuthentication ,
1.74 jmc 562: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 563: .Cm KerberosAuthentication ,
564: .Cm PasswordAuthentication ,
1.66 dtucker 565: .Cm PermitOpen ,
1.79 dtucker 566: .Cm PermitRootLogin ,
1.72 dtucker 567: .Cm RhostsRSAAuthentication ,
568: .Cm RSAAuthentication ,
1.66 dtucker 569: .Cm X11DisplayOffset ,
570: .Cm X11Forwarding ,
1.60 dtucker 571: and
1.66 dtucker 572: .Cm X11UseLocalHost .
1.33 dtucker 573: .It Cm MaxAuthTries
574: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 575: connection.
576: Once the number of failures reaches half this value,
577: additional failures are logged.
578: The default is 6.
1.1 stevesk 579: .It Cm MaxStartups
580: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 581: SSH daemon.
1.1 stevesk 582: Additional connections will be dropped until authentication succeeds or the
583: .Cm LoginGraceTime
584: expires for a connection.
585: The default is 10.
586: .Pp
587: Alternatively, random early drop can be enabled by specifying
588: the three colon separated values
589: .Dq start:rate:full
1.51 jmc 590: (e.g. "10:30:60").
1.53 jmc 591: .Xr sshd 8
1.1 stevesk 592: will refuse connection attempts with a probability of
593: .Dq rate/100
594: (30%)
595: if there are currently
596: .Dq start
597: (10)
598: unauthenticated connections.
599: The probability increases linearly and all connection attempts
600: are refused if the number of unauthenticated connections reaches
601: .Dq full
602: (60).
603: .It Cm PasswordAuthentication
604: Specifies whether password authentication is allowed.
605: The default is
606: .Dq yes .
607: .It Cm PermitEmptyPasswords
608: When password authentication is allowed, it specifies whether the
609: server allows login to accounts with empty password strings.
610: The default is
611: .Dq no .
1.62 dtucker 612: .It Cm PermitOpen
613: Specifies the destinations to which TCP port forwarding is permitted.
614: The forwarding specification must be one of the following forms:
615: .Pp
616: .Bl -item -offset indent -compact
617: .It
618: .Cm PermitOpen
619: .Sm off
620: .Ar host : port
621: .Sm on
622: .It
623: .Cm PermitOpen
624: .Sm off
625: .Ar IPv4_addr : port
626: .Sm on
627: .It
628: .Cm PermitOpen
629: .Sm off
630: .Ar \&[ IPv6_addr \&] : port
631: .Sm on
632: .El
633: .Pp
1.68 dtucker 634: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 635: An argument of
636: .Dq any
637: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 638: By default all port forwarding requests are permitted.
1.1 stevesk 639: .It Cm PermitRootLogin
1.38 jmc 640: Specifies whether root can log in using
1.1 stevesk 641: .Xr ssh 1 .
642: The argument must be
643: .Dq yes ,
644: .Dq without-password ,
1.52 jmc 645: .Dq forced-commands-only ,
1.1 stevesk 646: or
647: .Dq no .
648: The default is
649: .Dq yes .
650: .Pp
651: If this option is set to
1.52 jmc 652: .Dq without-password ,
1.1 stevesk 653: password authentication is disabled for root.
654: .Pp
655: If this option is set to
1.52 jmc 656: .Dq forced-commands-only ,
1.1 stevesk 657: root login with public key authentication will be allowed,
658: but only if the
659: .Ar command
660: option has been specified
661: (which may be useful for taking remote backups even if root login is
1.17 jmc 662: normally not allowed).
663: All other authentication methods are disabled for root.
1.1 stevesk 664: .Pp
665: If this option is set to
1.52 jmc 666: .Dq no ,
1.38 jmc 667: root is not allowed to log in.
1.46 reyk 668: .It Cm PermitTunnel
669: Specifies whether
670: .Xr tun 4
671: device forwarding is allowed.
1.47 reyk 672: The argument must be
673: .Dq yes ,
1.58 stevesk 674: .Dq point-to-point
675: (layer 3),
676: .Dq ethernet
677: (layer 2), or
1.47 reyk 678: .Dq no .
1.58 stevesk 679: Specifying
680: .Dq yes
681: permits both
682: .Dq point-to-point
683: and
684: .Dq ethernet .
1.46 reyk 685: The default is
686: .Dq no .
1.6 markus 687: .It Cm PermitUserEnvironment
688: Specifies whether
689: .Pa ~/.ssh/environment
1.9 stevesk 690: and
1.6 markus 691: .Cm environment=
692: options in
693: .Pa ~/.ssh/authorized_keys
1.9 stevesk 694: are processed by
1.52 jmc 695: .Xr sshd 8 .
1.6 markus 696: The default is
697: .Dq no .
1.9 stevesk 698: Enabling environment processing may enable users to bypass access
699: restrictions in some configurations using mechanisms such as
700: .Ev LD_PRELOAD .
1.1 stevesk 701: .It Cm PidFile
1.4 stevesk 702: Specifies the file that contains the process ID of the
1.53 jmc 703: SSH daemon.
1.1 stevesk 704: The default is
705: .Pa /var/run/sshd.pid .
706: .It Cm Port
707: Specifies the port number that
1.52 jmc 708: .Xr sshd 8
1.1 stevesk 709: listens on.
710: The default is 22.
711: Multiple options of this type are permitted.
712: See also
713: .Cm ListenAddress .
714: .It Cm PrintLastLog
715: Specifies whether
1.52 jmc 716: .Xr sshd 8
1.36 jaredy 717: should print the date and time of the last user login when a user logs
718: in interactively.
1.1 stevesk 719: The default is
720: .Dq yes .
721: .It Cm PrintMotd
722: Specifies whether
1.52 jmc 723: .Xr sshd 8
1.1 stevesk 724: should print
725: .Pa /etc/motd
726: when a user logs in interactively.
727: (On some systems it is also printed by the shell,
728: .Pa /etc/profile ,
729: or equivalent.)
730: The default is
731: .Dq yes .
732: .It Cm Protocol
733: Specifies the protocol versions
1.52 jmc 734: .Xr sshd 8
1.5 stevesk 735: supports.
1.1 stevesk 736: The possible values are
1.52 jmc 737: .Sq 1
1.1 stevesk 738: and
1.52 jmc 739: .Sq 2 .
1.1 stevesk 740: Multiple versions must be comma-separated.
741: The default is
742: .Dq 2,1 .
1.5 stevesk 743: Note that the order of the protocol list does not indicate preference,
744: because the client selects among multiple protocol versions offered
745: by the server.
746: Specifying
747: .Dq 2,1
748: is identical to
749: .Dq 1,2 .
1.1 stevesk 750: .It Cm PubkeyAuthentication
751: Specifies whether public key authentication is allowed.
752: The default is
753: .Dq yes .
754: Note that this option applies to protocol version 2 only.
755: .It Cm RhostsRSAAuthentication
756: Specifies whether rhosts or /etc/hosts.equiv authentication together
757: with successful RSA host authentication is allowed.
758: The default is
759: .Dq no .
760: This option applies to protocol version 1 only.
761: .It Cm RSAAuthentication
762: Specifies whether pure RSA authentication is allowed.
763: The default is
764: .Dq yes .
765: This option applies to protocol version 1 only.
766: .It Cm ServerKeyBits
767: Defines the number of bits in the ephemeral protocol version 1 server key.
768: The minimum value is 512, and the default is 768.
769: .It Cm StrictModes
770: Specifies whether
1.52 jmc 771: .Xr sshd 8
1.1 stevesk 772: should check file modes and ownership of the
773: user's files and home directory before accepting login.
774: This is normally desirable because novices sometimes accidentally leave their
775: directory or files world-writable.
776: The default is
777: .Dq yes .
778: .It Cm Subsystem
1.51 jmc 779: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 780: Arguments should be a subsystem name and a command (with optional arguments)
781: to execute upon subsystem request.
1.80 ! djm 782: .Pp
1.1 stevesk 783: The command
784: .Xr sftp-server 8
785: implements the
786: .Dq sftp
787: file transfer subsystem.
1.80 ! djm 788: .Pp
! 789: Alternately the name
! 790: .Dq internal-sftp
! 791: implements an in-process
! 792: .Dq sftp
! 793: server.
! 794: This may simplify configurations using
! 795: .Cm ChrootDirectory
! 796: to force a different filesystem root on clients.
! 797: .Pp
1.1 stevesk 798: By default no subsystems are defined.
799: Note that this option applies to protocol version 2 only.
800: .It Cm SyslogFacility
801: Gives the facility code that is used when logging messages from
1.53 jmc 802: .Xr sshd 8 .
1.1 stevesk 803: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
804: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
805: The default is AUTH.
1.27 markus 806: .It Cm TCPKeepAlive
807: Specifies whether the system should send TCP keepalive messages to the
808: other side.
809: If they are sent, death of the connection or crash of one
810: of the machines will be properly noticed.
811: However, this means that
812: connections will die if the route is down temporarily, and some people
813: find it annoying.
814: On the other hand, if TCP keepalives are not sent,
815: sessions may hang indefinitely on the server, leaving
816: .Dq ghost
817: users and consuming server resources.
818: .Pp
819: The default is
820: .Dq yes
821: (to send TCP keepalive messages), and the server will notice
822: if the network goes down or the client host crashes.
823: This avoids infinitely hanging sessions.
824: .Pp
825: To disable TCP keepalive messages, the value should be set to
826: .Dq no .
1.18 markus 827: .It Cm UseDNS
828: Specifies whether
1.52 jmc 829: .Xr sshd 8
1.40 jmc 830: should look up the remote host name and check that
1.18 markus 831: the resolved host name for the remote IP address maps back to the
832: very same IP address.
833: The default is
834: .Dq yes .
1.1 stevesk 835: .It Cm UseLogin
836: Specifies whether
837: .Xr login 1
838: is used for interactive login sessions.
839: The default is
840: .Dq no .
841: Note that
842: .Xr login 1
843: is never used for remote command execution.
844: Note also, that if this is enabled,
845: .Cm X11Forwarding
846: will be disabled because
847: .Xr login 1
848: does not know how to handle
849: .Xr xauth 1
1.15 jmc 850: cookies.
851: If
1.1 stevesk 852: .Cm UsePrivilegeSeparation
853: is specified, it will be disabled after authentication.
854: .It Cm UsePrivilegeSeparation
855: Specifies whether
1.52 jmc 856: .Xr sshd 8
1.2 stevesk 857: separates privileges by creating an unprivileged child process
1.15 jmc 858: to deal with incoming network traffic.
859: After successful authentication, another process will be created that has
860: the privilege of the authenticated user.
861: The goal of privilege separation is to prevent privilege
1.1 stevesk 862: escalation by containing any corruption within the unprivileged processes.
863: The default is
864: .Dq yes .
865: .It Cm X11DisplayOffset
866: Specifies the first display number available for
1.52 jmc 867: .Xr sshd 8 Ns 's
1.1 stevesk 868: X11 forwarding.
1.52 jmc 869: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 870: The default is 10.
871: .It Cm X11Forwarding
872: Specifies whether X11 forwarding is permitted.
1.13 stevesk 873: The argument must be
874: .Dq yes
875: or
876: .Dq no .
1.1 stevesk 877: The default is
878: .Dq no .
1.13 stevesk 879: .Pp
880: When X11 forwarding is enabled, there may be additional exposure to
881: the server and to client displays if the
1.52 jmc 882: .Xr sshd 8
1.13 stevesk 883: proxy display is configured to listen on the wildcard address (see
884: .Cm X11UseLocalhost
1.52 jmc 885: below), though this is not the default.
1.13 stevesk 886: Additionally, the authentication spoofing and authentication data
887: verification and substitution occur on the client side.
888: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 889: display server may be exposed to attack when the SSH client requests
1.13 stevesk 890: forwarding (see the warnings for
891: .Cm ForwardX11
892: in
1.19 jmc 893: .Xr ssh_config 5 ) .
1.13 stevesk 894: A system administrator may have a stance in which they want to
895: protect clients that may expose themselves to attack by unwittingly
896: requesting X11 forwarding, which can warrant a
897: .Dq no
898: setting.
899: .Pp
900: Note that disabling X11 forwarding does not prevent users from
901: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 902: X11 forwarding is automatically disabled if
903: .Cm UseLogin
904: is enabled.
905: .It Cm X11UseLocalhost
906: Specifies whether
1.52 jmc 907: .Xr sshd 8
1.1 stevesk 908: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 909: the wildcard address.
910: By default,
1.52 jmc 911: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 912: hostname part of the
913: .Ev DISPLAY
914: environment variable to
915: .Dq localhost .
1.8 stevesk 916: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 917: However, some older X11 clients may not function with this
918: configuration.
919: .Cm X11UseLocalhost
920: may be set to
921: .Dq no
922: to specify that the forwarding server should be bound to the wildcard
923: address.
924: The argument must be
925: .Dq yes
926: or
927: .Dq no .
928: The default is
929: .Dq yes .
930: .It Cm XAuthLocation
1.11 stevesk 931: Specifies the full pathname of the
1.1 stevesk 932: .Xr xauth 1
933: program.
934: The default is
935: .Pa /usr/X11R6/bin/xauth .
936: .El
1.55 jmc 937: .Sh TIME FORMATS
1.53 jmc 938: .Xr sshd 8
1.1 stevesk 939: command-line arguments and configuration file options that specify time
940: may be expressed using a sequence of the form:
941: .Sm off
1.7 stevesk 942: .Ar time Op Ar qualifier ,
1.1 stevesk 943: .Sm on
944: where
945: .Ar time
946: is a positive integer value and
947: .Ar qualifier
948: is one of the following:
949: .Pp
950: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 951: .It Aq Cm none
1.1 stevesk 952: seconds
953: .It Cm s | Cm S
954: seconds
955: .It Cm m | Cm M
956: minutes
957: .It Cm h | Cm H
958: hours
959: .It Cm d | Cm D
960: days
961: .It Cm w | Cm W
962: weeks
963: .El
964: .Pp
965: Each member of the sequence is added together to calculate
966: the total time value.
967: .Pp
968: Time format examples:
969: .Pp
970: .Bl -tag -width Ds -compact -offset indent
971: .It 600
972: 600 seconds (10 minutes)
973: .It 10m
974: 10 minutes
975: .It 1h30m
976: 1 hour 30 minutes (90 minutes)
977: .El
978: .Sh FILES
979: .Bl -tag -width Ds
980: .It Pa /etc/ssh/sshd_config
981: Contains configuration data for
1.53 jmc 982: .Xr sshd 8 .
1.1 stevesk 983: This file should be writable by root only, but it is recommended
984: (though not necessary) that it be world-readable.
985: .El
1.19 jmc 986: .Sh SEE ALSO
987: .Xr sshd 8
1.1 stevesk 988: .Sh AUTHORS
989: OpenSSH is a derivative of the original and free
990: ssh 1.2.12 release by Tatu Ylonen.
991: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
992: Theo de Raadt and Dug Song
993: removed many bugs, re-added newer features and
994: created OpenSSH.
995: Markus Friedl contributed the support for SSH
996: protocol versions 1.5 and 2.0.
997: Niels Provos and Markus Friedl contributed support
998: for privilege separation.
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